Journal article: This article essencially deals with an important point rarely tacked in the breakeven analysis, i.e. the taking into account of the incertainty which gets more and more important in our modem world. As far as transport infrastructure is concerned, uncertainty relates to traffic forecast, evolution with time of unit values and to the whole economic environment. This article analyses the impact of these different sorts of uncertainties and puts forward easy methods to take uncertainty into account in the usual calculations.